Degeneration of the nigral-striatal dopaminergic pathway is a major characteristic of patients with Parkinson's disease. Presently the etiology of this progressive degenerative process in unknown. However, the symptoms of this disorder can apparently be remedied by increasing neurotrasmitter levels in the surviving neurons. This is accomplished by administering high levels of the precursor L-Dopa. The present investigation is also an attempt to increase endogenous dopamine neurotransmission by increasing the activity of the rate-limiting enzyme tyrosine hydroxylase. Hopefully this can be accomplished by raising the in vivo tissue levels of tetrahydrobiopterin (BH4). Our approach was to administer large doses of BH4 (2.0, 5.0 and 10.0 mg/kg) to patients while monitoring physiological responses as indicated in the protocol. Concurrent with this, CSF biopterin levels, DOPAC and HVA will be determined where possible. Our initial results with BH4 administration to Parkinson's patients revealed no significant effect on their normal motor functions, while their CSF level of BH4 was increased 4-5 fold.